This invention relates to a brake unit including a master cylinder containing at least one master-cylinder piston, a booster piston the diameter of which is larger than that of the master-cylinder piston for displacing the master-cylinder piston, a pressure-control valve for controlling pressure from a power source, and a coupling between the booster piston and a brake pedal. A brake unit of this type is known from the British Pat. No. 1,105,351.
Practice has shown that it is advantageous for obtaining smoothness of operation of a vehicular brake system if the generation of an increasing braking force requires an increasing brake pedal travel. This desired dependence between generated brake pressure and pedal travel results in a brake system automatically if the brake pressure is generated in a master cylinder by the driver's pedal effort alone. If the brake pressure is generated by introducing controlled pressure from a power source, in principle only a short control travel has to be overcome to generate the full braking pressure. If it is nevertheless desired to obtain a brake-pressure responsive pedal travel, additional measures are necessary.
In the brake unit of the above-cited British Patent, a brake-pressure responsive pedal travel is obtained by rigidly coupling the brake pedal with a booster piston. The pressure-control valve is activated through lever means. While this known brake unit offers the advantage of a brake-pressure responsive pedal travel, it will have disadvantageous effects if the power source fails. As long as the power source is available, the master-cylinder piston will be displaced by the force acting on the booster piston. This force is substantially generated by the pressure from the power source. If the controlled pressure introduced is at its maximum, the displacement stroke of the master-cylinder piston is likewise very large so that the brake pedal is at its permissible maximum. If the power source fails, the master-cylinder piston can only be displaced by the driver's pedal effort alone. However, since this force is by far smaller than the force exertable by the power source on the booster piston, the master-cylinder piston can be displaced only a small amount. From this ensues that only a low pressure builds up in the brake system. The maximum permissible pedal travel cannot be made use of since the force applied to the brake pedal is insufficient.